This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Our long-term goal is to understand the roles of protein phosphatases in regulating p53 signaling pathway during DNA damage response and tumorigenesis. In this proposal, we will concentrate on the regulation of MdmX-p53 auto-regulatory feedback loop by protein phosphatases. MdmX inhibits the transcriptional activity of the tumor suppressor p53 that is induced by DNA damage or oncogenic stresses. Overexpression of MdmX and aberrant p53 regulation lead to deregulated cell growth and contribute to tumorigenesis. High levels of MdmX associate with 24.6% of various human cancers. In particular, MdmX is overexpressed in 18.5% of human colorectal cancers. Thus, MdmX must be tightly controlled in cells. Recently, we have identified Wip1, a ser/thr protein phosphatase, as a homeostatic regulator of MdmX. Our recent studies demonstrated that Wip1 directly dephosphorylates MdmX at the ATM-targeted Ser403. Wip1 inhibits the DNA damage-induced ubiquitination and degradation of MdmX, leading to the stabilization of MdmX and reduction of p53 activities. In addition to Wip1, three isoforms of Protein Phosphatase 1 (PP1) have also been identified as positive regulators for MdmX. In light of these exciting findings, we hypothesize that protein phosphatases may play a physiological role in the regulation of the MdmX-p53 pathway. Hence, we will use in vivo and intro approaches to systematically investigate this previously untested hypothesis in this proposal.